I won't hold my breath

In English we have the saying I won't hold my breath...For example, your friend says she will call you and you know she will not so in reply to her saying she will call you, you say I won't hold my breath.

I looked up to hold one's breath and I found trattenere il fiato...

If I answered an Italian in the same way would it make sense?
could I say: Non trattenero' il fiato?

I was wondering if anyone had a clearer translation for "I won't hold my breath". A friend keeps putting our meeting off and he said "quando torno a Milano ci sentiamo" and I want to say 'I won't hold my breath.'
"campa cavallo" seems a bit old fashioned or am I mistaken? I want to say it as a joke but also give him a little nudge! So if I say "non ci spero molto/troppo", I don't want it to sound to harsh!